This invention relates to a high yield synthesis of polyol fatty acid polyesters, sucrose polyesters in particular, via transesterification. More specifically, this invention relates to a process having an improved purification or separation step for obtaining polyol fatty acid polyester, particularly sucrose polyesters.
The food industry has recently focused attention on polyol polyesters for use as low calorie fats in food products. See, for example, U.S. Pat. No. 3,600,186 and U.S. Pat. No. 3,963,699. As a result of this attention, there is a current need for a high yield synthesis of polyol fatty acid polyesters. Historically, such syntheses have been conducted using a mutual solvent to solubilize a polyol and esters of long-chain fatty acids, thus providing a homogenous reaction medium suitable for catalytic transesterification. One variation of this process, known as the Snell synthesis, has been employed as a means for preparing both poly- and lower-esters. However, the solvents heretofore employed in such processes are difficult to separate from the final product and are inappropriate for food uses, therefore limiting the usefulness of such synthesis in the foods industry. Accordingly, recent efforts have been directed toward the discovery of a high yield synthesis of polyol fatty acid polyesters which does not employ such solvents or provides complete separation.
Other solvent-free transesterification processes are known in the art. For example, U.S. Pat. No. 3,251,827 discloses the preparation of sucrose polyesters by means of a solvent-free interesterification using phenyl esters. However, phenol is liberated during the reaction. Phenol is also inappropriate for food use and is difficult to separate. Accordingly, this process does not satisfy current needs for a synthesis of polyol fatty acid polyesters for use in the foods industry.
Feuge, et al, "Preparation of Sucrose Esters by Interesterification", Journal of the American Oil Chemical Society, 47(s), 56-60 (1970), disclose a single stage solvent-free transesterification useful in synthesizing fatty acid esters of sucrose. However, this process is limited to the synthesis of lower esters. It has been experimentally determined that if the sucrose/methyl ester ratio of the Feuge et al reaction is lowered by use of excess methyl esters in an effort to synthesize polyesters, the reactants will disproportionate and precipitate sucrose which then caramelizes to form a brittle, charred waste product. Furthermore, the Feuge et al article reports low yields using lower alkyl esters. The more successful Feuge et al synthesis uses fatty acid methyl carbitol esters as starting materials. Unfortunately, methyl carbitol is, itself, inappropriate for food use. Thus, the Feuge et al process also fails to satisfy current needs for a synthesis of polyol fatty acid polyesters useful in the foods industry.
More recently, polyol fatty acid polyesters, particularly sucrose polyesters, useful in the foods industry have been prepared in high yields without solvents via transesterification by heating a mixture of a polyol, such as sucrose, a fatty acid lower-alkyl ester, an alkali metal fatty acid soap, and a basic catalyst selected from alkali metals, alkali metal alloys, alkali metal alkoxides and alkali metal hydrides at a temperature of 110.degree.-180.degree. and pressures of 0.1 to 760 mm Hg for a time sufficient to form a homogenous melt of partially esterified polyol and unreacted starting material, then adding excess fatty acid lower alkyl esters to the reaction product previously made to form the polyol fatty acid polyesters, and, finally, separating the polyol fatty acid polyester from the reaction mixture; see U.S. Pat. No. 3,963,699. While this process is appropriate for the foods industry and prepared polyol fatty acid polyester in a high yield with low contamination, improvements in the separation of polyol fatty acid polyester are of interest to avoid the formation of intractable emulsions, handling losses and liquid-solid separations. Further, the solvent extraction purification of the prior art does not remove color-forming impurities and it is therefore necessary to bleach the polyol fatty acid polyester with a clay. The use of a bleaching clay adds the problems of filtration and solids handling to an otherwise easily mobile liquid product with its accompanying product losses, capital expense and processing time. Accordingly, a separation, purification and recovery process for polyol fatty acid polyesters which does not have these problems would be highly desirable.
It is, therefore, an object of this invention to provide a process for the preparation of polyol fatty acid polyesters which produces such product substantially free of alkali metal fatty acid soaps and color-forming impurities.
It is a further object of this invention to provide a process for separation and recovery of polyol fatty acid polyesters in which acid/caustic refining and bleaching can be omitted.
Still a further object of this invention is the provision of an improved process for producing polyol fatty acid polyesters in which the improvement provides a separation step with simpler processing equipment, low cost reagents, high recovery of product and little, if any, emulsion formation.
These and other objects will be readily apparent from the following description of the improved process of the invention.